Dengue (DENV) has re-emerged in the last 25 years as one of the most significant emerging infectious diseases worldwide, posing a significant category A biodefense threat to the United States with ongoing epidemics in Puerto Rico, and recent outbreaks in Hawaii (2001) and Texas (1999, 2005, 2006). Its global resurgence, while partly explained by changing human demographics, increasing hyperendemicity, and mosquito vector expansions, remains poorly understood, especially with respect to the importance of virus evolution. We fill this critical gap by conducting a comparative evolutionary examination of DENV strains undergoing epidemic expansion, from replicate populations across the Asia-Pacific arena, paired with directed tissue and animal model assays for changes in virus phenotype related to epidemics. Our long-term goal is to resolve how viral genetic change drives disease emergence. The objective of the proposed research is to characterize evolution in dengue viruses isolated from epidemics relative to their endemic forms and confirm the impact of genetic changes in experimental models, the rationale being that identification of such viral genetic changes and their mechanisms of fixation will illuminate determinants of dengue epidemic potential and ultimately pathogenesis. Specific Aim 1: Determine the evolution of DENV between endemic and epidemic transmission phases to identify synapomorphies of higher epidemic potential. Specific Aim 2: Confirm the relationship between genetic changes in DENV epidemic strains and their phenotypic effects in in vitro models. Specific Aim 3: Confirm the relationship between genetic changes in DENV epidemic strains and their phenotypic effects in in vivo models